This invention relates to pipe coating generally and, more particularly, to a method of coating pipe characterized by a longitudinally extending weld seam.
The size of pipe used in construction of pipelines has steadily increased until diameters in the range of 50-60 inches are now common. Pipe of extremely large diameters may presently be formed only through utilization of a longitudinal weld which extends the length of the pipe. The resulting weld bead projects above the remainder of the pipe surface by a distance of up to about 1/2 inch.
When pipe coating procedures employing side extrusion equipment are utilized for coating longitudinally welded pipe, the weld bead causes special problems. A typical procedure for side extrusion coating of pipe is disclosed in U.S. Pat. No. 3,823,045 issued to Ralph Hielema on July 9, 1974. The techniques heretofore utilized to fill in the area adjacent the weld bead have been to increase the amount of coating material in the area by increasing the speed of the extruder or depositing a powder material on the pipe which is sintered by the heat of the pipe immediately ahead of the extrusion coating application. These techniques are described in detail U.S. Pat. Nos. 3,972,761 and 3,814,646, respectively.
There are several disadvantages to the techniques disclosed in the above referenced patents including difficulty in applying the required amount of filler material, the need to employ relatively expensive and complicated sensing systems, and the limitation of requiring that the pipe be at a high temperature if a sinter coating is to be used. In some instances, it has been found that a superior coating results if the pipe is not heated to the extremes required for sinter coatings but, instead, is at relatively low temperatures in the vicinity of the ambient air when the coating is applied.
It is, therefore, a principal object of the present invention to provide a method of coating the area adjacent the weld bead of longitudinally welded pipe which does not required the pipe to be at high temperatures as a prerequisite to the application process.
Another object of this invention is to provide a method of coating the area adjacent the weld bead of longitudinally welded pipe which does not require expensive sensory equipment for sensing the location of the weld bead as the pipe is rotated.
A further aim of the invention is to provide a method of coating the area of longitudinally welded pipe adjacent the weld bead which may easily be adapted to pipe of different diameter.
Still another object of the invention is to provide a method of coating the area adjacent the weld bead of longitudinally welded pipe which is not affected by minor imperfections in the pipe surface or out-of-round pipe conditions.
It is also an important objective of this invention to provide a method of coating the area adjacent the weld bead of longitudinally welded pipe whereby the area adjacent the weld bead may be coated ahead of an extrusion coating operation so as to preclude the possibility of any interference with the extrusion process.
Other objects of the invention will be made clear or become apparent from the following description and claims, when read in light of the accompanying drawings.